Digital electronic safety and arming system

ABSTRACT

An acceleration signal is integrated over a determinable time period to develop a digital count which is a function of velocity; that signal is then again integrated over an identical time period to produce a digital count as a function of the distance traveled during the acceleration periods. The distance signal is compared to a reference signal to determine whether a missile, for example, undergoing the detected acceleration has traveled a sufficient distance from its launching site, or vehicle such as an aircraft, to be safely armed for detonation. A digital distance signal equal to the reference signal affects arming of the missile by aligning an explosive train, for example, for detonation. First and second interlocks in the system are responsive to a ready-to-launch signal and an initial minimum acceleration signal, respectively, to ready the system for arming. The final step of arming is accomplished by a cam driven into arming position by simultaneously actuated drive members which are pivotably attached to the arming cam. If only one of the two drive members is actuated by a spurious signal, for example, the cam is pivotably displaced and locked into a non-armed position.

United States Patent Anderson et al.

[ June 24, 1975 1 DIGITAL ELECTRONIC SAFETY AND ARMING SYSTEM [75]Inventors: Matthew E. Anderson. Ontario;

Stephen L. Redmond, China Lake, both of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: Oct. 29, 1973 [21'] App]. No.: 410,922

Related US. Application Data [62] Division of Ser. No. 255,746, May 22,1972, Pat. No.

[52] US. Cl. l02/70.2 R; 102/76 [51] Int. C13. F42C 9/00; F42C 11/00;F42C 15/40 [58] Field of Search lO2/70.2 R, 76; 73/490, 73/503 [56]References Cited UNITED STATES PATENTS 2,885.503 5/1959 Smith 102/70.2 R2.889.777 6/1959 Rabinow 102/702 R 3,028,550 4/1962 Naydan ct 211....3,500,746 3/1970 Ambrosini 102/702 R 3,672.302 6/1972 Shaw l02/70.2 R3,750,583 8/1973 White ct a1. 102/70.2 R

Primary E.raminerRobert F. Stahl Assistant Examiner-C. T. JordanAttorney. Agent, or FirmR. S. Sciascia [57] ABSTRACT An accelerationsignal is integrated over a determinable time period to develop adigital count which is a function of velocity; that signal is then againintegrated over an identical time period to produce a digital count as afunction of the distance traveled during the'acceleration periodspThedistance signal is com pared to a reference signal to determine whethera missile, for example, undergoing the detected acceleration hastraveled a sufficient distance from its launching site. or vehicle suchas an aircraft, to be safely armed for detonation. A digital distancesignal equal to the reference signal affects arming of the missile byaligning an explosive train, for example, for detonation. First andsecond interlocks in the system are responsive to a ready-to-launchsignal and an initial minimum acceleration signal, respectively, toready the system for arming. The final step of arming is accomplished bya cam driven into arming position by simultaneously actuated drivemembers which are pivotably attached to the arming cam. If only one ofthe two drive members is actuated by a spurious signal, for example, thecam is pivotably displaced and locked into a non-armed position.

10 Claims, 6 Drawing Figures r Jr 19 f 2/ C 5- A x116 Ila 9 ELECTRICCIRCUIT l7 ta/raw i CIRCUIT PATENTEDJuN24|s1s 3. 890.901

SHEET 1 ELECTRIC CIRCUIT --1 ARM/N6 CIRCUIT FIG.1

PATENTEDJUN24 ms 3,890,901

SHEET 2 -slllIlllll'iili1ili-| COUNT COUNT UP DOWN -lllllllllllll*iiH|.l

COUNT COUNT UP DOWN PATENTEnJuM24 ms 3,890,901

SHEET 3 ACCELEROMETER \28 A OSC'LLATOR I2 BIT UP/OOWN COUNTER 42 L L fr49 IMHZ CLOCK FULL ACCUMULATOR ADDER 22 BIT SR.

CARRY ADDEND SEQUENCER DELAY 22 317' SR 50 INTEGRATION SEQUENCE 44LAUNCH SWITCH \4, REGISTER 22 BIT s.R.

FIG. 2

2 D REO/sTER 48- 5I\ T COMPARATOR 5s UP/DOWN i COMMAND DIGITALELECTRONIC SAFETY AND ARMING SYSTEM This is a division, of applicationSer. No. 225,746, filed May 22, 1972 now US. Pat. No. 3,793,890.

BACKGROUND OF THE INVENTION Both prior art and current safety and armingdevices as used on missiles, for example, attempt to employ some form ofdouble integration of detected missile acceleration in order todetermine safe arming distance beyond which the missile may be safelydetonated. Such systems customarily use a mechanical device to performthe iterative integration of missile acceleration. While mechanicaldevices are relatively rugged, safe, and fairly inexpensive, they alsohave inherent problems and disadvantages. Because such integrationdevices are essentially mechanical in nature they are inherently sufferundesirable effects from friction, and vibration which detracts fromboth their accuracy and reliability. Moreover, such mechanical devicesincur problems with lubrications an and require relatively extreme closetolerances in their manufacture, assembly, and operation.

The cummulative result of these and other disadvantages is that it isnot uncommon for safety and arming systems using mechanical integrationdevices to have to undergo several individual adjustments to enabletheir performance within required tolerances. With such devices usualaccuracies for arming distances are of the order of to of the actualsafe separation distance. Moreover, nearly all acceleration drivenintegration devices are employed in prior art and present safety andarming systems are only pseudo acceleration double integrators for thereason that mechanical integrators will only provide a reasonablyaccurate measure of arming distance if the acceleration forces, i.e.,the boost levels, remain fairly constant and above about 6 gs inmagnitude. This is due mainly to the customary use of a runawayescapment to retard the initially driven arming masses. When missileacceleration approaches zero of become negative, the escapement controlmechanism no longer continues to integrate the acceleration signals toobtain separation distance but simple stop moving.

Thus, problems have been encountered with the prior art mechanicalintegrating devices when it is attempted to employ them for other thantheir specifically designed purpose. For example, when the boostduration time is very short or the acceleration-time profile is notadequate, the prior art mechanical integrating devices simply will notfunction to provide sufficiently accurate output signal. Some effort hasbeen undertaken to develop mechanical devices that will perform a trueacceleration double integration function. However, it is questionablewhether the size, cost, accuracy, and ruggedness requirements such asare typical of a missile safety and arming system can be fully met bysuch a mechanical device operating on very small forces.

Accordingly, it is highly desirable that a safety and arming system bedevised applying electronic techniques rather than mechanical devices toperform inte gration operations upon an acceleration signal derived froman accelerometer to obviate some of the more troublesome disadvantagesof prior art systems having a comparable objective of measuring safearming distance from detected measurements of acceleration.

Moreover, it is highly desirable that such electronic techniques beadapted for use with digital signals and digital logic rather thananalog electronic signals for the reason that digital techniques areamenable to the inclusion of fail-safe logic in the system so thatextraneous, erroneous, or external signals can be detected so as toprevent inadvertent arming of the device by initiating procedures toeither reset or dud the missile in which the safety and arming system isemployed.

SUMMARY OF THE INVENTION In addition to incorporating fail-safeelectronic logic as an inherent feature of the present invention to minimize a premature arming signal which may result from extraneous voltagesdue to high radiation sources, noise, or other unwanted electricalinterference, the present invention includes multiple safety features.

The complete safety and arming system of the present invention primarilyincludes basic sub-assemblies comprising (I an accelerometer fordeveloping a signal as a function of the acceleration it undergoes, (2)electronic logic circuitry including apparatus for producing a digitaloutput representative of velocity in response to an initial accelerationsignal as well as logically arranged electronic elements for performingiterative integration on digital signals, and (3) a fail-safe armingmechanism.

In the system there is an arming path, operative upon actuation, fordetonating the missile. Such a path m? typically comprise, an explosivetrain, which may be actuated by an electric detonator connected toreceive a signal such as a target detection indication confirming that aguidance system has horned in on a target. The arming path is renderedsafe from inadvertent detonation by a spurious signal, for example,through the technique causing a discontinuity in the path, such as maybe given effect, for example, by the lateral displacement of a portionof an explosive train so that the missile is not truly armed until thelaterally displaced portion is realigned to provide a continuous armingpath.

In a typical system this discontinuity could be provided by an armingslider member which includes the laterally displaced portion of theexplosive train. In a preferred embodiment of the present invention thearming slider member is retained in the laterally displaced position torender the explosive train inoperative and retain the missile in itsunarmed state by two interlocks; the first of the interlocks may takethe form of a launch interlock such as a member which is solenoidoperated and can only be withdrawn from its interlock position retainingthe arming slider member by the actuation of an intent-to-launch switch,for example, which impresses electrical energy upon the solenoid andremoves the intent-to-launch interlock from a detent in the armingslider member.

In a preferred embodiment of the present invention the sameintent-to-launch interlock (which incidentally operates independently ofthe second interlock) can also be made to release the accelerometer froma lock condition under which it is inoperative to respond toacceleration forces and enable an electrical circuit reponsive toaccelerometer signals. The same actuation caused by closure of theintent-to-launch switch can perform the additional function of applyingpower to the missile system.

A second independently operative interlock comprises a minimum boostsensor which, in its normal position at rest, interlocks in a detent ofthe arming slider member preventing its movement laterally andmaintaining the discontinuity in the explosive train so that the missileis not armed. Upon launch of the missile, a minimum boost sensor, whichmay take the form of a spring-loaded mass disposed to respond to thelaunch acceleration of the missile, is displaced by a predetermined andprecalculated minimum boost or acceleration, removing the interlock fromthe arming slider member so that the slider member is in a condition tobe laterally moved against its spring loading force for aligning theexplosive train in a continuous path and thus arming the missile. Thearming slider member, however, is normally retained in the unarmedposition by a suitable spring bearing against one of itsends.

The accelerometer, having been enabled by closure of theintent-to-launch switch, responds to the acceleration forces developedby launching of the missile generating an electrical signal as afunction of such acceleration forces by means of, for example, apotentiometer having a variable tap carried on the accelerometer andmoved relative to the resistive element of the potentiometer. In thepreferred embodiment of the pres ent invention a variable tap isnormally at the center of the resistive element of the potentiometerdeveloping a balanced signal and movement of the accelerometer inresponse to acceleration forces causes an unbalanced signal to bedeveloped by the potentiometer.

The signal thus developed in response to acceleration forces is employedin electronic logic circuitry to develop a digital signal over apredetermined time increment. The digital signal is thus the timeintegral of acceleration and representative of velocity attained. Thedigital velocity signal is then integrated to provide a digital signalindicative of the distance traveled by the missile. The digital distancesignal is compared to a reference signal for determination of whetherthe missile has traveled a predetermined safe arming distance. If thedigital distance signal is equal to the predetermined reference signal,the missile is at a safe distance from the launch vehicle to be armed.

The final step in arming the missile is achieved by actuating afail-safe mechanism which aligns the explosive train. The fail-safefeature of this final step is that the arming mechanism is such as torequire two precisely simultaneous signals in order to arm the missile.If only one signal is generated, due to a spurious false signal source,the mechanism is such that it will be retained in a non-arming conditionthereby preventing arming of the missile in response to such spurious orfalse signals as may be developed inadvertently in electronic circuitry.

The safety and arming system of the present invention employs a uniquemethod and means for developing a digital signal representative ofvelocity which is generated from an acceleration signal provided by theaccelerometer. An up-down digital counter is arranged to be under thecontrol of a square wave, counting up when the square wave is positiveand down when the square wave is not positive. When the square wave issymmetrical having positive and non-positive portions of equal duration,the net count for the full cyclic period or time increment of the squarewave cycle is zero because the up-down counter first counts up and thendown an equal number of counts. The zero net cumulative digital count isnormally the situation when there is no acceleration.

However, upon launch the acceleration which is detected is employed tocause a commensurate state of imbalance between the duration of thepositive portion of the cycle of the square wave generator and theduration of the non-positive portion of the square wave generator. As aresult, the up-down counter which the square wave generator controls iscaused to count up in excess of the down count during the predeterminedtime increment, subtracting the down-count from the up-count so that thenet cumulative count in the digital counter at the end of each cyclicperiod of the multivibrator is a digital count of the time integral ofacceleration, representative of the velocity achieved during the periodof acceleration. The acceleration forces have been effectively measuredover a time period thereby integrating the measured acceleration toyield a digital output representative of velocity.

This unique method and means of employing an acceleration signal todevelop a digital output signal which is a measure of velocity achievedduring the period of acceleration, has a number of advantages inaddition to the fact that its output is in digital form. One of theseadvantages is the fact that the concept is capable of developing adigital output which is negative in character; that is to say, thedown-counts may be in excess of the up-counts during a particular timeincrement so that the resultant negative count is indicative of negativeacceleration or deceleration and a resultant loss in velocity. Thenegative digital signal can then be subtracted from a prior velocitysignal to ascertain the current velocity in digital terms Accordingly,the concept and teachings of the present invention embrace multipleadvantages, several of which reside in sub-assemblies of the system,apart from the overall system itself.

It is a primary object of the present invention, therefore, to developan improved method and means to provide a digital output signal derivedfrom an acceleration signal over a determinable time incrementrepresentative of the time integral of such acceleration as a digitalmeasure of velocity.

An equally important object of the present invention is to provide fordigital electronic iterative integration of an acceleration signal forgenerating a digital signal as a measure of distance traveled during theperiod of acceleration.

A further important object of the present invention is to provide asignificantly improved safety and arming system employing digitalelectronic techniques.

Yet another important object of the present invention is to provide adigital electronic safety and arming system including multiple safetyinterlocks which are independent of each other and are responsive toseparate, independent, actuation forces.

Another most important object of the present invention is to provide adigital electronic safety and arming system in which the final step ofarming is achieved by a fail-safe procedure.

A further object of the invention is to provide a failsafe armingprocedure which will abort the arming when spurious; false signals aredeveloped and lock the arming device in a non-arming condition.

These and other features, objects, and advantages of the presentinvention will be better appreciated from an understanding of theoperative principles of a preferred embodiment as described hereinafterand as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. I is a partially cross-sectional, schematic illustration of thedigital electronic safety and arming system of the present invention indifferent stages of its operation;

FIG. 1a is an illustration of the operation of one of the fail-safefeatures of the present invention;

FIG. 2 is a schematic block diagram of the digital electronic portion ofthe system of the present invention;

FIG. 3 is a schematic wiring diagram of the oscillator employed in thedigital electronic portion of the invention; and

FIGS. 4a and 4b are illustrations of the type'of' waveforms generated bythe oscillator of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 schematicallyillustrates an embodiment of the digital electronic safety arming systemof the present invention in its initial and unarmed state, indicated bythe solid line representation of its movable elements. The final stateand positions of movable elements resulting from the arming sequence isshown by dash-line representation.

The unarmed state of the system is caused by a discontinuity in anarming path which, in this case, may take the form of a laterallydisplaced portion 11 of an explosive train, for example. The laterallydisplaced portion 11 of the explosive train 10 is maintained in theunarmed position by a spring 12 and two interlocks received into twodifferent detents l3 and 14 of an arming slider 15. The displacedportion 11 of the arming path 10 prevents detonation of the arming pathbeyond the arming slider 15.

Therefore, the system (as shown by the solid lines depicting itselements) may be said to illustrate its unarmed condition. The firstinterlock comprises a solenoid actuated rod 16 which performs severalfunctions in addition to its interlocking function with the detent 13.In its non-actuated position as shown by the solid line representationin FIG. 1, the interlock assembly 16 maintains a power switch 17 in anopen condition as well as retaining a variable tap 18 from contactingthe stationary resistive element 19 of a potentiometer thereby avoidingthe development of an unwanted spurious, signal.

The solenoid 20 is actuated by the closure of switch 21, as shown by thedash lines, which delivers electrical energy to the coil of the solenoid20 causing the rod 16 of the interlock means to move in an upwarddirection to the position indicated by dash lines where it is thereafterretained against gravity and other acceleration forces byelectromagnetic force due to the continuing current flow through switch21. The upward movement of rod 16, as shown by the dash lines, withdrawsthe end of the rod interlock 16 from the interlock detent l3 and at thesame time closes the switch 17, as indicated by its dash linerepresentation, for connecting power to the remainder of the missilesystem. The same actuation through member 16a moves the variable tap 18to the position represented by the dash lines into contact with thestationary portion of a potentiometer which is in the form of theresistive element 19. The switch 21 may be referred to as anintent-to-launch switch and is actuated in an aircraft. for example. asa step necessary prior to launching the missile.

When the missile is launched, assuming an upward direction relative tothe drawing of FIG. I, acceleration forces are developed which operateupon the second interlock 22 having a rod 23 supported on the mass 24whihch is urged upwardly by the spring 25. The mass of element 24 androd 23, and the force of spring 25 are pre-calculated together withfrictional forces, so that a predetermined minimum acceleration or boostmust be achieved by the missile in order that the mass travel along ahelical path 26 translate past the lock-out detent 27 to the positionindicated by dash line representation. This is the second step in thearming sequence of the present system and it should be noted that the'two interlocks are independently operative and respond to separate anddifferent actuating forces.

The accelerating forces also operate upon an accelerometer 28 carryingthe variable tap 18 which is in contact with the stationary resistiveelement 19 of a potentiometer. When the accelerometer 28 is at rest,i.e., not subject to acceleration forces, the tap 18 is normally in thecenter of the resistive element 19. However, as the accelerometer 28responds to accelerating forces, it causes the tap 18 to be moved alongthe potentiometer away from its mid-point to generate an unbalancedsignal as a function of the acceleration. The unbalanced signal operatesin conjuction with electronic circuitry 29 to develop a digital signalwhich is the time integral of the acceleration and therefore representsthe velocity developed by the missile during the period of acceleration.

The electronic circuitry 29 then integrates the digital velocity signalto provide a digital signal which is a function of the distance traveledby the missile. The digital distance signal is compared in an armingcircuit 30 with a reference signal representing a predetermined safedistance at which the missile may be armed. When the safe distance isreached, as determined from the fact that the digital distance signalequals the reference signal, the arming circuit 30 delivers simultaneoussignals to control means, such as transistors 31 and 32, applying powerto dual arming devices which may take the form of the solenoids 33 and34 as illustrated.

Simultaneous actuation of solenoids 33 and 34, as shown by dash lines,drives an arming cam 35 forward, as indicated by dash lines, positioningthe arming slider 15 to its armed position as represented by dash lines,so that the discontinuity in the explosive train 10 is eliminatedbyrealignment of the originally laterally disposed portion ll. Thecontinuity of the arming path 10 is maintained by the retention of thearming cam 35 in its actuated position by appropriate lock detents 36and 37, which move to their locking position represented by dash lines,and also by the interlock l6 dropping into a third detent 38 in thearming slider 15.

One of the features and advantages of the present invention is thefail-safe nature of the arming mechanism by reason of which it requiresprecisely simultaneous signals from the arming circuit to effectuate thefinal arming step of the missile.

If, for example, a spurious or false signal is developed in theelectronic integrating circuitry 29, or the arming circuit 30, a singlesolenoid will be actuated. This results in the rotational pivoting ofthe arming cam 35 as illustrated in FIG. 1a so that it does not performits arming function but rather is locked into an unarmed condition byeither the detent 36 being received into recess 39 or the detent 37being received into a recess 40, (as shown in FIG. 1a), depending uponwhich way the arming cam is rotated. The direction of such pivotalmovement is determined by which one of the two arming solenoids isactuated, and, as can readily be appreciated, the arming sequence isaborted and the arming cam 35 locked in an unarmed position.

The digital electronic integrating circuit represented at 29 in FIG. 1is shown in the more detailed schematic illustration of FIG. 2. Anup-down counter 41 receives two inputs; one input is developed by theclock 42 which, in one embodiment of the present invention, wasoperative at I Ml-Iz; the other input is generated by an oscillator 43which, in a typical embodiment, was operative at a frequency of I KI-Iz.Oscillator 43 is used to develop a square wave output, a positiveportion of which controls the counter to count the clock pulsesadditively up and the non-positive portion of the square wave beingemployed to cause the counter to count down, subtractively. Theoscillator 43 is of such design that it is responsive to a signaldeveloped by the accelerometer 28 of FIG. 1 displacing the variable tap18 along the stationary resistive element 19.

The oscillator 43 generates square wave output signals of constantfrequency, i.e., equal duration cyclic periods, but having positive andnon-positive square wave portions varying in the duration in accordancewith the signals developed by the accelerometer 28 in its effect uponthe tapped signal produced by positioning the variable tap 18 on theresistive element 19. That is to say, that when the accelerometerdevelops no signal, the oscillator will generate a square wave havingpositive and non-positive portions of equal duration, thus causing thecounter 41 to count up and down an equal number of constant frequencypulses received from clock 42 so that its net cumulative digital countis zero in response to the zero acceleration.

However, when the accelerometer is responsive to acceleration forces anddevelops an output signal indicative thereof, the oscillator 43 willdevelop positive excursions of the square wave which are longer in dura-I tion, while the non-positive portions of the square wave arediminished by a commensurate period of time. Thus, the up-down counter41 is caused to count a commensurate number of clock pulses up and downso there is a net cumulative digital pulse count produced because theup-down counter functions so that the number of down counts within acyclic period effectively subtracts from the up counts previouslyaccumulated during the same cyclic period. Since the frequency ofoscillator 43 remains constant, there is a known determinable timeincrement of one cyclic period over which the up-down count has beenmade. The net digital up-down count generated in response toacceleration over a known time increment in the case of the illustrationof FIG. 2 would be 1 millisecond since the oscillator 43 is operative atl KHz. The net count remaining in the counter 41 at the end of the timeincrement therefore is the time integral of the acceleration, orvelocity achieved during that time increment. The count is thendelivered to the V register 44 and an accumulator 45.

In accordance with the concept of the present invention the separationdistance from the launch vehicle is computed by using an iterativeintegration technique. The missile velocity relative to the launchvehicle is available from the up-down counter 41 as previouslyexplained; therefore, only one successive integration is necessary toCompute distance.

The technique employed within the concept of the present inventionassumes that the acceleration is con stant over one time perioditeration. Since the time period iteration is relatively very small as,for example, the l millisecond period as developed by the oscillator 43in the embodiment illustrated in FIG. 4, a further assumption may bemade that the change in distance during the iteration period is equal tothe average of the former velocity (one time iteration earlier) and thepresent velocity multiplied by the time increment. This may be expressedas:

u+l (VII MP4) l It then is apparent that only two additions arenecessary to calculate distance for comparison with a reference signalwhich is representative of a predetermined safe distance for arming,provided that the predetermined reference sigrial is scaled by the value2/At.

The electronic circuits necessary to carry out these calculations areillustrated schematically in FIG. 2. A MHz clock 42 which may be of thesolid state crystal type provides constant frequency signals to anup-down counter 41, a sequencer 46 develops signals for controlling thesequence of operations of the calculating portion of the circuitry whichincludes four shift registers, the accumulator 45, the added register47, V register 44, and the (2/At) D,, register 48. In a typicalembodiment each of the four shift registers previously mentioned mayhave a 22 bit capacity or the equivalent of a decimal capacity of about4 X 10.

In the combination of apparatus employed in the embodiment illustratedin FIG. 2, a distance measurement capacity of about 2,000 feet isprovided, i.e.,

The accumulator 45'and the V,, register 44 are parallel input/ serialoutput types because they are required neously changed in opposite sensewithin cyclic periods of constant duration, and such changes in thedurations of its two different values are generated as a function of theinput signal developed by the accelerometer 28.

The oscillator 43 is illustrated in FIG. 3 and operates in the followingmanner. Transistors 51 and 52, together with resistive elements 53 and54, and capacitors 55 and 56, comprise a free-running multivibratoroscillator having the bases of the transistors connected to a commonresistive element 19 which, in turn, is connected to a B-lvoltagethrough a variable tap 18.

A third stage of the circuit illustrated in FIG. 3 comprises atransistor 57 which is an up-down command control connected to theup-down counter 41.

When the circuit, as illustrated in FIG. 3, is in the condition wherevariable tap 18 is at the center of the resistive element 19, a squarewave will be developed at a frequency of l KHZ wherein the crossover ofthe waveform from a first value, such as a positive voltage, to a secondvalue, such as zero voltage, will occur at the middle of each cyclicperiod as is illustrated in FIG. 4a. However, when the accelerometer 28of the arming system is actuated by acceleration forces and consequentlymoves the variable tap 18 along the stationary resistive element 19, astate of imbalance is created and results in a square wave of the samefrequency, but whichis imbalanced in the sense that the positive signalhas a longer duration than before and the crossover where the squarewave changes from positive to negative-going condition is displaced fromthe midpoint within the cyclic period of the signal, as is illustratedin FIG. 46. Thus, the command signal delivered by the control transistor57 controls the sense of the count of up-down counter 41 so that itcounts up at a constant rate for a greater duration of time than itcounts down, thereby leaving a net cumulative count within such cyclicperiod.

On the other hand, if the accelerometer 28, having been initiallyactuated by acceleration forces, then undergoes subsequent decelerationforces, a state of imbalance is developed in the other sense, generatinga net negative count, so to speak, representative of the diminishingvelocity occurring during the period of deceleration. Such signal,representative of a diminishing velocity, may then be subtracted fromthe prior positive velocity, and the remainder is a digital signalrepresentative of net current velocity.

When there is no acceleration, the circuit of FIG. 3 will be in a stateof balance so that the up-down counter 41 will be controlled to count upand down for equals periods of time and therefore produce a zero netdigital count indicative of zero velocity. In its operation, the circuitof FIG. 3 is arranged so that when the acceleration signal is applied tothe resistive element 19, the resistance on one side of the oscillatorwill increase and on the other side of the oscillator it will decrease.Both these changes are linear and therefore the frequency and periodiccycle of the oscillator remain the same; however, the duration of thetime during which the oscillator is generating a positive output voltagediffers from the duration of time when it is generating a zero voltageoutput, and such difference is a function of the magnitude ofacceleration. Obviously the greater the amplitude of acceleration, thelarger the time difference will be between these two conditions andaccordingly the larger net cumulative count will be developed by theup-down count.

Since this count is developed over each cyclic period of the oscillatorof FIG. 3, it is representative of the time integral of accelerationwhich is equal to velocity. Thus, up-down counter 41 develops a signalwhich is a measure of velocity for each time increment, i.e., eachrepetitive cyclic period of the oscillator 43. As previ ously described,such net cumulative count at the end of each cyclic period orpredetermined time increment is delivered from the up-down counter 41 tothe accumulator 45 and also to the V register 41.

The sequence and steps of operation of the digital electronic circuitryis controlled by a sequencer 46 connected to deliver command sequencesignals to the electronic circuitry of FIG. 3 in the following manner.

The clock pulses, as derived from the l Ml-l z clock 42, V

are connected to the sequencer which is programmed to repetitivelydevelop a repetitive sequence of signals in the following order. Theinitial commands of the sequencer 46 cause transfer of the content ofthe updown counter 41 to the accumulator register 45 and also to the V,,register 44. This places the most recent value of velocity, asrepresented by a net, digital count, in a position to enter into thecomputations for a particular iteration.

Then a sequence command is given to shift right enable the adder 49, astandard single bit serial adder which is used for all additions. Aserial adder may be employed rather than a parallel type, because it isless expensive and its relatively slow add time of operatio" can betolerated for this type of function. The numbers to be added aredelivered from the accumulator register 45 and the addend register 47,received by the adder 49 one bit at a time, and the sum output of theadder 49 is recirculated into the accumulator register 45. This stepadds the previous value of velocity to the most recent value of velocityto give a new average velocity (scaled by 2).

Next, the sequencer 46 gives the command to trans fer the contents ofthe D register 48 to the addend register 47. This prepares thepreviously calculated value of separation distance (as scaled by Z/At)for summing with the average velocity. Then, the adder 49 receives ashift right enable signal which adds the previous value of accumulatedseparation distance (scaled by 2/At) to the new average velocity.

The next sequence command transfers contents of the accumulator register45 to the D, register 48 which stores the currently calculated value ofseparation distance (scaled by 2/At) for use in the next iterationduring the next successive time increment.

The next step in the sequence of commands developed by the sequencer 46causes a comparison of the digital value in the D register 48 with thepredetermined value or digital count which is representative of apre-calculated safe arming distance. This is accomplished in comparator51 and when the digital count (scaled) transferred from the D register48 is equal to the predetermined digital count (also scaled)representative of a pre-calculated safe arming distance stored incomparator 51, an arming signal is delivered as an output of comparator51.

The next successive step commanded by the se quencer 46 is the transferof the contents of the V,, register 44 into the addend register 47. Thisstep stores the current velocity measurement for computation use in thenext iteration.

Appropriate resetting of the sequencer 46 is then accomplished. Afterthe completion of an iteration cycle, the previously described sequencewill be restarted by the sequencer 46, the sequence of the previouslydescribed steps of calculation having been completed in less than onecyclic time period of the oscillator 43 which is l millisecond. Upon thebeginning of a new iteration period, the computations are repeated withthe new value of calculated missile velocity and the updated value ofdistance.

In order to determine if the computed distance is equal to thepre-calculated safe arming distance, a comparison is made for eachiterative time period. If desired, only the most significant bit in theD,, register 48 may be checked and if it is a logical one," then anarming signal may be initiated. However, if more accuracy is required,more of the most significant bits in the D,, register 48 can be comparedwith the digital signal stored in the comparator 51 which isrepresentative of the pre-calculated safe arming distance.

The pre-calculated safe arming distance may be inserted into thecomparator 51 immediately prior to launch which would allow thepredetermined safe arming distance to be changed in accordance with anyparticular situation or special requirement.

A technique for preventing premature arming of the missile may beemployed by checking the most significant bit position of the D,,register 48 during each clock output pulse from the sequencer 46. If thebit which is checked in the D,, register 48 is a logic l at any timeduring the computation sequence, (except when a number is being enteredinto the D, register 47, or when an actual comparison is being made)then it may be assumed that an extraneous, spurious signal has appeared.

In that event, the system will deliver but a single signal to only oneof the two arming actuators, such as the solenoids 33 and 34, dependingupon which one is connected to the output of the comparator 51. Thearming cam 35 will therefore be pivotally displaced, aborting arming ofthe missile and locking the arming cam 35 in a non-arming condition asis illustrated in FIG. 111.

If, however, the proper signals for arming are developed in accordancewith the operation of the system, there will be a logic 1 pulse fromboth the comparator 51 and from the sequencer 46 into the armingactuators 33 and 34, simultaneously acting upon the arming cam 35 to armthe missile in the manner previously de scribed by realigning the armingpath to eliminate its discontinuity.

One of the more significant advantages of the digital electronic safetyarming system of the present invention is that it is capable ofperforming double integration upon an acceleration signal to determineseparation distance in response to any missile acceleration timeprofile, including the case of negative acceleration which can occurduring missile flight. By contrast all known, currently used mechanicalsafety and arming systems are not capable of operating with low ornegative accelerations.

Further because the present invention does not use any mechanicalcomponents in its acceleration double integration calculations, itvirtually eliminates all inaccuracies due to friction, out of toleranceparts, lubrication problems, inertia, vibrations, and other potentialproblems associated with mechanical devices.

By employing digital logic to iteratably integrate acceleration, a highorder of accuracy can be achieved. Inherently, the accuracy of computedseparation distance in the use of the system of the present inventiondepends principally upon the accurracy of the accelerometer employed andvery little upon the digital circuitry.

For example, employing a reasonably accurate integration technique and arelatively small iteration time of the order of l millisecond, theelectronic processing and calculation of the present invention is veryprecise. If an accelerometer with an accuracy of i 3 percent is used, itis readily feasilble that the calculated separation distance accuracywill be better than i 5 percent. This is in comparison to the averagemechanical integrating system of the prior art wherein the accuracy isof the order of i 25 percent.

A further advantage in the use of digital logic which is inherent in theconcept of the present invention is the availability of integratedcircuits to perform logic functions. Integrated circuits are small,rugged, reliable, and, as is well known to those skilled in thepertinent arts, can operate over a broad required temperature range of,for example, -55 to C.

A further important feature of the use of digital logic in the form ofintegrated circuits is their potential low cost. As integrated circuitsare coming into greater and greater demand, production will be increasedresulting in a decreased cost per unit as has been reliably pre dictedby the electronics industry.

A significant advantage of the concept of the present invention is thatit readily lends itself to many alternative embodiments and the use ofdifferent elements as designed within its concept and teaching. Forexample, a rocket motor pressure sensor rather than a G-time sensorcould be used to insure that a minimum boost had occurred, and thus beemployed to remove the second interlock from the arming mechanism.

Another alternative within the concept and teaching of the presentinvention is that a launch ejection sensor could be used other than anelectronic solenoid so as to actuate the first interlock and the armingsequence would thus be started upon the indication that launch hadoccurred.

Additionally, fail-safe logic could be readily adapted and used in thedigital circuitry for zeroing all the registers if extraneous orincorrect voltages were detected at any point in the calculation.

If desired and permitted by cost considerations, a doubly safe systemcould be readily devised employing dual independent accelerometers andintegrator circuits.

Further, many appropriate additional sources of information such asminimum air velocity, for example, or a signal indicative of the factthat a missile guidance system had become operative could be employed inan AND gate of the arming logic to further enhance safety.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A digital electronic safety and arming system for an explosivemissile comprising:

an arming path operative upon actuation for detonating said missile;

means for causing a discontinuity in said arming path;

a first interlock engaged with said means for maintaining saiddiscontinuity;

a safety release responsive upon actuation for disengaging said firstinterlock;

a second interlock engaged with said means and being responsive to apredetermined minimum acceleration for disengagement;

means for producing a digital output signal representative of distancein response to an acceleration signal developed during a determinablefixed time period;

comparator means for comparing said output signal with adigitalreference signal representative of a minimum safe distance, andproducing an arming signal when the compared signals are equal; and

arming means responsive to said arming signal for eliminating saiddiscontinuity thereby arming said missile.

2. A digital electronic safety and arming system for a missile asclaimed in claim 1 wherein said first interlock is operative to maintainsaid accelerometer in disabled condition and said safety release isoperative to enable said accelerometer.

3. A digital electronic safety and arming system for a missile asclaimed in claim 2 wherein said safety release is operative to connectpower to the missile electrical system.

4. A digital electronic safety and arming system for a missile asclaimed in claim 1 including means for retaining said missile in itsarmed condition.

5. A digital electronic safety and arming system for a missile asclaimed in claim 4 wherein said means for retaining said missile in itsarmed condition is said first interlock means in an ancillary interlockposition of engagcment.

6. A digital electronic safety and arming system for a missile asclaimed in claim 1 wherein said arming path is an explosive train.

7. A digital electronic safety and arming system for a missile asclaimed in claim 6 wherein said means for causing a discontinuity insaid arming path is a laterally displaced portion of said explosivetrain.

8. A digital electronic safety and arming system for a missileas'cl'aimed in claim 7 wherein said arming means is an arming camresponsive to drive means actuated by said arming signal for returningsaid laterally displaced portion of said explosive train to a positionof alignment restoring the continuity of said arming path.

9. A digital electronic safety and arming system for a missile asclaimed in claim 8 wherein said drive means comprise dual drive memberspivotally attached to said cam for requiring simultaneous actuation ofsaid dual drive members to effect alignment of said arming path.

10. A digital electronic safety and arming system for a missile asclaimed in claim 9 including locking means operative to retain saidarming cam in a non-armed position when said cam is pivotally rotated bythe actuation of only one of said dual drive members.

1. A digital electronic safety and arming system for an explosive missile comprising: an arming path operative upon actuation for detonating said missile; means for causing a discontinuity in said arming path; a first interlock engaged with said means for maintaining said discontinuity; a safety release responsive upon actuation for disengaging said first interlock; a second interlock engaged with said means and being responsive to a predetermined minimum acceleration for disengagement; means for producing a digital output signal representative of distance in response to an acceleratioN signal developed during a determinable fixed time period; comparator means for comparing said output signal with a digital reference signal representative of a minimum safe distance, and producing an arming signal when the compared signals are equal; and arming means responsive to said arming signal for eliminating said discontinuity thereby arming said missile.
 2. A digital electronic safety and arming system for a missile as claimed in claim 1 wherein said first interlock is operative to maintain said accelerometer in disabled condition and said safety release is operative to enable said accelerometer.
 3. A digital electronic safety and arming system for a missile as claimed in claim 2 wherein said safety release is operative to connect power to the missile electrical system.
 4. A digital electronic safety and arming system for a missile as claimed in claim 1 including means for retaining said missile in its armed condition.
 5. A digital electronic safety and arming system for a missile as claimed in claim 4 wherein said means for retaining said missile in its armed condition is said first interlock means in an ancillary interlock position of engagement.
 6. A digital electronic safety and arming system for a missile as claimed in claim 1 wherein said arming path is an explosive train.
 7. A digital electronic safety and arming system for a missile as claimed in claim 6 wherein said means for causing a discontinuity in said arming path is a laterally displaced portion of said explosive train.
 8. A digital electronic safety and arming system for a missile as claimed in claim 7 wherein said arming means is an arming cam responsive to drive means actuated by said arming signal for returning said laterally displaced portion of said explosive train to a position of alignment restoring the continuity of said arming path.
 9. A digital electronic safety and arming system for a missile as claimed in claim 8 wherein said drive means comprise dual drive members pivotally attached to said cam for requiring simultaneous actuation of said dual drive members to effect alignment of said arming path.
 10. A digital electronic safety and arming system for a missile as claimed in claim 9 including locking means operative to retain said arming cam in a non-armed position when said cam is pivotally rotated by the actuation of only one of said dual drive members. 